In general, various types of signal-measuring apparatus have been developed to measure various characteristics of electronic signals, for instance, selection level meters, spectrum analyzers, network analyzers and the like.
FIG. 1 shows a typical arrangement of a conventional selection level meter employing a heterodyne system.
A basic operation of the heterodyne type selection level meter is as follows.
An input signal to be measured, which is supplied from an input terminal 101, is mixed in a mixer 103 with an oscillator output signal of a local oscillator 102, provided within the measuring apparatus. An intermediate signal (referred to as an "IF signal") is output from mixer 103. That is to say, the heterodyneconverted intermediate signal is bandwidth-limited by a resolution bandwidth filter 104 and furthermore detected by a detector 105. The detected signal is converted into a digital signal by an analog-to-digital converter 106, signal-processed in a data processing section 107, and thereafter displayed on a CRT display section 108.
FIG. 2 shows a typical arrangement of a conventional network/spectrum analyzer employing a heterodyne system. An operation of the network/spectrum analyzer will now be summarized.
The network/spectrum analyzer includes an R (reference signal) channel input terminal 109 and a T (test signal) channel input terminal 110. The input signals to be measured which are input from these two channel terminals are input to corresponding first and second mixers 112 and 113. These input signals are mixed with oscillator output signals of a common internal local oscillator 111 in these mixers 112 and 113. That is to say, the heterodyne conversion is performed in mixers 112 and 113. The outputs of mixers 112 and 113 are bandwidth-limited by first and second resolution bandwidth filters 114 and 115, and then detected in first and second detectors 116 and 117. The outputs of first and second detectors 116 and 117 are converted into digital signals in an analog-to-digital converter 118, and furthermore signal-processed in a data processing section 119, and finally displayed on an CRT display device 120. When measuring the phase difference between both input signals, the input signals in both the signal channels are first filtered in resolution bandwidth filters 114 and 115 and then supplied to phase detector 121 to obtain the phase difference between these input signals as an analog voltage. This analog voltage is converted into a digital signal by analog- to-digital converter 118. After the digital signal is processed in data processing section 119, it is displayed on CRT display device 120.
These prior art signal-measuring apparatus has a drawback, however, in that, as shown in FIG. 3, a filtering center frequency (F.sub.1) of the resolution bandwidth filter is varied due to the temperature drift of this filter and the aging effects. That is to say, a detuning phenomenon occurs in the conventional apparatus. As illustrated in FIG. 3, the filter characteristic of the resolution bandwidth filter is, in general, influenced by the temperature drift and/or aging effects.
In other words, errors of (F.sub.2 -F.sub.1) and (L.sub.2 -L.sub.1) exist in the peak (center) frequencies and the signal levels, respectively, under the condition that the characteristic curve G.sub.1 represents the normal filter condition and the characteristic curve G.sub.2 indicates the detuned filter condition.
To correct these errors in the conventional apparatus, an oscillator (not shown) having stable and synthesized oscillating frequencies and stable output signal level is prepared in addition to the conventional measuring apparatus. For example, the oscillator is driven in such a manner that the frequency produced by local oscillator 102 of the measuring apparatus shown in FIG. 1 is swept to measure the detuning curve G.sub.3 as represented in FIG. 4. Thereafter, the filter output level "L.sub.3 " at a predetermined frequency "F.sub.3 " in the curve "G.sub.3 " is read, and a difference (L.sub.3 -L.sub.0) between this level "L.sub.3 " and a preset level "L.sub.0 " of the synthesized oscillator is obtained as a correction value. Thus, the error (L.sub.3 -L.sub.0) caused by the detuning is corrected by utilizing this correction value according to the conventional correcting method.
In accordance with such a conventional detuning correcting method, an external reference signal source must be employed and complex calibration steps are also required.
Moreover, as can easily be understood from the detuning curve G.sub.3 of FIG. 4, the calibration data is obtained at a given point on the left slope of the curve G.sub.3 , not at a peak point thereof, so that stability of the corrected level data is still impaired because of the detuning phenomenon.
In addition, the serious drawback of a level shift may occur, which is caused by not only the resolution bandwidth filter, but also the circuit elements other than the bandwidth filter, with the result that the errors cannot be compensated for when determining the measuring results.
When the phase difference characteristic between two input signals is measured in the circuit arrangement of the conventional network/spectrum analyzer shown in FIG. 2, another error may be contained in the measurement result of the phase difference characteristic, since first and second resolution bandwidth filters 114 and 115 in the respective signal processing channels are detuned.